1. Field of the Invention
The present invention relates to image forming apparatuses employing electrophotography process (for example, copying machines, laser printers). More specifically, the present invention relates to an improvement of toner replenishment for a developing apparatus employing two-component developer containing toner and carriers.
2. Description of the Background Art
In an image forming apparatus employing electrophotography process, toner concentration in a developer contained in a developing apparatus for developing latent electrostatic images must be kept in a constant appropriate range. When the toner concentration in the developer becomes too low, the density of the toner image will be lowered. Therefore, toner replenishing systems have been proposed in which a prescribed amount of toner is newly supplied when it is detected that the toner concentration in the developer or the toner image density becomes lower than a prescribed level, so as to maintain the toner concentration in a proper range.
More specifically, a magnetic sensor for detecting magnetic permeability of the developer is provided in the developing apparatus to detect a change in the magnetic permeability of the developer incidental to the change of the toner concentration. Based on the result of detection, when the toner concentration is lower than a reference value, a prescribed amount of toner is newly supplied. Alternatively, a latent electrostatic image of a reference pattern is formed on a photoreceptor, and the density of the toner image formed by developing the latent electrostatic image is detected by a photo sensor. When the detected density becomes lower than a predetermined value, a prescribed amount of toner is newly supplied.
Generally, the timing of detection of the toner concentration and of the replenishment is constant regardless of the types of originals and the manner of copying.
In the above described toner replenishment, a prescribed amount of toner is supplied periodically for the following reason. Namely, the toner concentration in the developing apparatus is not uniformly increased in real time in direct response to the new supply of the prescribed amount of toner.
However, if the timing of the toner replenishment is determined such that the toner is newly supplied after the toner is stirred for a prescribed time period, the toner supply will be late to cope with the rapid consumption of toner.
Namely, when an original having large image area must be copied for many times continuously, the toner consumption is large and a small supply of toner cannot cope with the decrease of the toner concentration.
To solve the above problem, the amount of toner to be newly supplied may be increased to a large amount.
However, when the large amount of toner is consumed in a short period of time and the toner replenishment is repeated for many times, the newly supplied toner cannot be uniformly dispersed. Consequently, the fluctuation of the toner concentration becomes large and aside therefrom, some part of the toner is not fully electrified because of insufficient frictional electrification with carriers. This may possibly result in fog on the copy. The toner which is not fully electrified may splash in the apparatus causing contamination.